3/4 supply.....1/2 section.....3/4 run....

Just a quickie. I thought I saw this answer before somewhere, but now I can't seem to find it again.

My question is one of fluid dynamics:

I have a 1/2" (copper) supply coming into my bathroom that changed from 3/4", inside the wall about 4". Impossible to reach without destroying adjacent bath. It was feeding just a shower in this room, but now I want to feed a shower and bath so I need it to be 3/4" for another 15 ft or so....

Can a 3/4 supply line have about a 1 foot section of 1/2" pipe in it without any noticeable drop in pressure or volume?

Plumber buddy says "No sweat". Just put reducer coupling at accessible point and go back to 3/4" for remaining run....."Absolutely NO drop in performance"

Of course I'm here for a second, third, and fourth opinion because this will be feeding a multiple head shower and I'd hate to regret it later.....

Referring tohttp://www.irrigationtutorials.com/copperloss.htm
and assuming worst case that
a) the 1/2" length is Type L copper (type M is thinner and thus larger in ID) and
b) the flow rate is 5 GPM (very hot shower at very high volume),
the pressure loss in 1 foot of 1/2" is less than 0.2 psi.
Since a typical 3/4" metered town water supply presents at least 50 psi at 25GPM to a single family home,
such a loss should not be noticeable.

Since 1 foot of 1/2" type L has a volume of .012 gallons,http://www.copper.org/applications/plumbing/techref/cth/tables/cth_table2b.htm
the velocity of the water at 5GPM is
(5/.012) = 417 feet per minute or about 7 feet per second, well beyond the 5 feet per second recommended as a working limit and likely quite noisy. Of course at 2.5 GPM, the velocity is 3.5 feet per second, still perhaps noisy but with no sidewall erosion dangers.

So, you want a lot of water, and as little pipe noise as possible. Very normal. I've never met a person who wanted the opposite.

Friction losses are a linear sum of the distance. A short distance of narrower pipe is not a long distance. Everyone keeps posting the same info, worded differently. Your plumber friend is right, within reason. To go deeper, just FYI, it is irrelevant where the narrow piece is in the entire run; it is independent.

So, a short distance doesn't add a lot of resistance to your flow.

It is true that it is more like a nozzle -- and no-one can predict the noises it will produce when "stressed" to its maximum limit, even if one could be there and see all the shapes (bends in of the pipe). Any direct contact to studs or other structure will transmit noise through solid material and then into the air where you can then hear it. So, it you can slide a thin pad between the pipe and its strapping, that will help a lot.

Musical instruments are complex geometry. Even the temperature of your hot water will change the noise the pipe produces. Personally, I think that the shape of the 3/4" pipe right after the 1/2" segment is what will determine a good part of the possible noise. Are you aware of how water produces microscopic vapor bubbles when it is pressurized and flowing fast over a microscopically rough surface like the inside of a copper pipe? This produces a hiss noise or lisping whistle. This noise gets amplified (slightly) at the 3/4" aperture acting like a horn spout does. When these bubbles come out of the 1/2" pipe, they won't get absorbed back into the main stream of water right away, but will at first expand a bit since the pressure has dropped. These two sudden changes will make some form of music or noise too, which then travels inside the water to an elbow or bend and then comes out into the air there. The length of the straight pipe determines at least one of the harmonics in the noise. That's the hypothesis, which you can verify by building a batch of similar setups and measuring noise as you change one variable at a time. If you were so inclined.

If anyone has more to add about pipe noises, please feel free to contradict me; I am eager and willing to learn what ever you know.

Now, Mr. Baum, if you really want to know whether you can fan out to three nozzles running 2.5 GpM each, and have acceptable plumbing performance elsewhere too, here is a way to test before installing. Build a pipe run equivalent (in number of elbows, and in total length, and in 3/4" copper) to the one you intend to install in your walls, and run it into the tub. Time how long it takes to fill the tub to e.g. 60 gallons, while turning on all the water (tub and shower, flushing toilet too). Note if you do not install the shower nozzles in this simulation as you are looking to know the total maximum amount of water you can get, you will not have the ultimate end flow, since a shower nozzle is a restriction, a friction, which reduces flow too. But I wouldn't sweat it. You can repeat this filling experiment with and without all the other variables.

summary:
1. More length = more loss = less water.
2. More bends = more loss = less water.
3. Shape of bends is one factor in creating noise.
4. Padding to prevent direct physical contact is important.

Since noises can start to happen later, it is good to pad even if there is no noise.

What size line is feeding the house now and what material is it? If you have a PRV I would increase the pressure as high as you can get it. With 3 heads running at the same time you will need as many # pressure as you can get and the small piece of copper may affect it.

What size is your water heater. At 7.5 GPM your hot water won't last long...maybe 10 min. or less depending on time of year and the temp setting of the water heater.

I think I will try to change this small section of pipe, just to be safe.

I will try to get to the tee in the wall without disturbing the adjacent bath. I will heat up the existing tee and pull it off the 3/4 supply. I can then either try to solder another tee (not very likely, but I'll try), or I bought a shark-bite tee that may work, but it's kind of bulky and this is a really tight space.

i have used Pex-Al-Pex (very malleable, and keeps its shape) to run a new line through a tight space with no access. You can use this to make a new run back to the HW heater or close to it.

Aqua (Kitec) Pex-Al-Pex pipe has a huge __HUGE!__ diameter, far greater than 3/4" pex and also greater than 3/4" copper. I could find its true dimensions somewhere on the company web site and post them here, if this needs to be known precisely.

Since the pipe is a three-layer composite, with aluminum as the middle layer, it keeps its shape when you straighten it or bend it. This is a good feature when you need to send it through a tight space.

Water flows quietly in 1.) a large I.D. pipe, that has 2.) smooth inside walls and 3.) gentle flowing curves. Since the pipe can be bent into curves, you don't have abrupt elbow changes of direction / plane.

A brand new run back to your HW heater is also good for your shower since it supplies water on a dedicated circuit.

david
p.s. if you remove the 1/2" or the Tee you are 99% committed. Hope this alternative gets to you before too late.

Consider bypassing the 1/2" bootleneck with a run a new pipe at 3/4" OR even 1" diameter (@ $3 a foot in Type L copper nowadays---PEX looks better and better). Pick the exit and reentry points into the existing line to be those easiest to work at. The water flow (7.5 GPM??) will favor the path presenting the lower total psi drop.